Four important considerations in the design of an antenna are bandwidth, beamwidth, profile height, and ability to transceive radiation of specified polarization(s). The requirements of an antenna in these four categories depend on the application for which the antenna is designed. In a number of applications, including Global Positioning System (GPS) arrangements, UHF satellite communications, and certain military uses, a relatively large bandwidth, wide beamwidth, low profile, and ability to transceive circularly polarized radiation are advantageous. For example, if an antenna is to be mounted on a high performance vehicle and used for UHF satellite communications, it should have a low profile for aerodynamic efficiency, and it must have the bandwidth, beamwidth, and polarization characteristics necessary to communicate with UHF satellites. Crossed-slot antennas have been used in such situations because of their generally low profile and ability to be driven in phase quadrature to achieve circular polarization.
Typically, crossed-slot antennas comprise a rectangular cavity, dimensioned to be resonant at the frequency of operation. Crossed slots are cut into one broadwall of the cavity to form the radiation means. The cavity is typically excited via probes inserted into the cavity or by transmission lines place across the slots.
One type of crossed-slot antenna is shown in FIG. 1. Each of the four planar patches of metallization is short circuited on a different edge of the ground plane. Thus, radiating edges of the four patches form crossed slots across the antenna as illustrated in FIG. 1. Also, the radiating edges of diagonally opposed patches are collinear and define two nearly continuous and crossed apertures in conjunction with the ground plane below. In certain applications, it is desirable to provide balanced transmission of signals (equal power and 180 degrees out of phase) by the diagonally opposed patches so as to maintain symmetry in the radiation pattern of the antenna. However, this has normally required an expensive and bulky network to feed the antenna patches.
For example, in order to use a crossed-slot antenna to transceive circularly polarized radiation, the patches must be fed in phase quadrature and, preferably, with equal power. In order to accomplish this, a full quadrature (0, 90, 180, 270 degrees) hybrid, such as the one illustrated in FIG. 1, is typically used to feed the patches. Hybrid networks of this type are normally large and complicated, and tend to be relatively narrowband. Either two 90 degree hybrids and a 180 degree hybrid, or a 90 degree hybrid and two 180 degree hybrids (as shown in FIG. 1) are generally used. The necessity of such complicated hybrid networks increases both the cost and the bulk of the antenna.